Field of the Invention
The invention relates to a method for manufacturing a photocatalyst based on titanium dioxide containing carbon or metal in a pulsation reactor.
Technological Background of the Invention
Photocatalytic materials are semiconductors in which, when exposed to light, electron-hole pairs are formed that generate highly reactive free radicals on the material surface. Titanium dioxide is a semiconductor of this kind. It is known that, by being irradiated with UV light, titanium dioxide can remove natural or artificial impurities in air and water in that the impurities are decomposed and/or oxidized (mineralized) to form environmentally friendly end products. One disadvantage of titanium dioxide is that the visible component of sunlight does not trigger any photocatalytic activity.
However, various procedures are known for modifying titanium dioxide in such a way that it can also utilize the visible component of sunlight in order to develop photocatalytic activity—e.g. doping or surface-coating of the titanium dioxide crystals with carbon, or with metal ions, such as Fe, V, Cr, etc.
WO 2005/108505 A1, for example, discloses a process for producing a carbon-containing, titanium dioxide-based photocatalyst that is photoactive in visible light. The process is based on a parent titanium compound, present in the form of an amorphous or semi-crystalline titanium oxide, hydrous titanium oxide, titanium hydrate or titanium oxyhydrate. The parent titanium compound is present in the form of either a fine-grained solid or a suspension and has a BET surface of at least 50 m2/g. The parent titanium compound is mixed with a carbon-containing substance and subsequently heat-treated at up to 400° C. The heat treatment leads, on the one hand, to the formation of crystalline titanium dioxide and is intended, on the other hand, to induce a surface reaction of the carbon-containing compound in the form of partial decomposition and modification of the titanium dioxide particle surface.
The product manufactured contains carbon in quantities of 0.5 to 4% by weight, in a surface layer and on the surface of the titanium dioxide particles, and preferably display a specific surface area of 100 to 250 m2/g.
According to WO 2005/108505 A1, the heat treatment is performed in heat treatment units, such as rotary kilns, fluidized-bed reactors, fluidized-bed driers or heated ploughshare mixers, in continuous mode, or sometimes also in batch mode. Although good product qualities can be obtained with these processing methods, it is not possible to achieve consistent product quality, i.e. reproducibility, over extended periods of time. The quality fluctuations are due to the temperature profiles and wide residence-time distributions occurring in the heat treatment unit. If temperature peaks occur at certain points in the heat treatment unit (“temperature hotspots”), uncontrolled degradation reactions of the organic compounds take place, up to the point of complete combustion of the organic compounds, as a result of which the photoactivity of the product, on the one hand, and its color, on the other, are impaired by intercalated carbon black. Moreover, agglomerates are easily formed, as a result of which complete combustion of the organic compounds can take place across particles in the event of temperature hotspots occurring.
For manufacturing metal-containing, specifically iron-containing, TiO2 photocatalysts, EP 0 666 107 B1 discloses a method where an aqueous solution, containing a titanium compound and an iron compound, is hydrolyzed. The product is subsequently dried, or calcined at temperatures of up to 500° C.
WO 2012/139726 discloses a method for manufacturing iron-containing TiO2 photocatalyst, where an aqueous suspension of titanium oxyhydrate nanoparticles and iron(III) ions is taken as the starting point, iron(III) hydrate is subsequently precipitated, and the particles are finally subjected to heat treatment (preferably at 100° C. to 400° C.).
Both methods are based on a precipitation process that cannot be controlled uniformly and reproducibly, and thus cannot lead to a homogeneous product.
There is therefore a need for a method for manufacturing photocatalysts based on titanium dioxide containing carbon or metal, with the help of which consistently good product qualities can be produced, even over an extended period of time.